1. Field of the Invention
The present invention relates to an electronic musical instrument which can control the timbre of a musical tone using a filter.
2. Description of the Related Art
A conventional electronic musical instrument, such as an electronic organ, is designed to control the timbre of a musical tone using an analog filter.
Recently, a digital filter has been used in various types of electronic apparatuses.
In an acoustic apparatus, such as a compact disk player (hereafter referred to as "CD player"), or a digital audio tape player (hereafter referred to as "DAT player"), the digital filter serves to cut aliasing noise.
The digital filter may also be used in the electronic musical instrument to control the timbre of a musical tone.
Digital filter are classified into two types; a finite impulse response type (hereafter referred to as "FIR type") and an infinite impulse response type (hereafter referred to as "IIR type").
The FIR type digital filter is conventionally used in the acoustic apparatuses; this filter may also be employed in the electronic musical instruments.
In order to acquire a desirable filter property, however, the FIR type digital filter theoretically needs execution of an infinite series of delays, thereby requiring a great number of filter coefficients. If the filter is designed approximately, 256 stages of delays have to be done, and a very large number of filter coefficients are required accordingly.
Although the FIR type digital filter can be used in acoustic apparatuses, therefore, its use in electronic musical instruments would be difficult.
The acoustic apparatuses use a constant signal sampling frequency of a signal, so that there needs only one frequency response of the filter for eliminating the aliasing noise. It does not therefore matter much in this case if many filter coefficients are necessary.
On the other hand, the electronic musical instruments require that a plurality of timbres be switched from one to another from time to time; the filter coefficients should also be switched from time to time accordingly. Many filter coefficients would disturb the smooth selection, and further complicate the filter structure.
As a solution to this problem, the IIR type digital filter may replace the FIR type because the former digital filter has fewer filter coefficients than the latter.
The IIR type digital filter still requires a considerable number of filter coefficients, though reduced, to accurately express the timbres. The use of the IIR type digital filter cannot therefore completely overcome the aforementioned shortcoming.
Further, the digital filter has an inherent rough characteristic, thereby deteriorating the smoothness of musical tones.